1. Field of the Invention
This invention relates to transparent films which can be rendered translucent by stretching and rendered transparent again by releasing the film. The invention also relates to a method of making such films.
2. Description of the Prior Art
Transparent films comprised of blends of crystalline polymers and various constituents are known. U.S. Pat. No. 3,132,027 teaches the preparation of transparent films suitable for packaging by melt-coating the article to be packaged with a blend of a polymer having greater than 30 percent crystallinity with up to 20 percent of an oil and rapidly quenching the coating. The patent further teaches that only when quenching is rapid (as when melt-coated articles are plunged into an ethanol bath at -25.degree. C.) are transparent films obtained. When the melt-coated article is allowed to cool slowly, the coatings obtained are undesirably cloudy and intolerably fragile.
Transparent films of a blend of mineral oil and polyolefin are described in U.S. Pat. No. 3,201,364. This patent teaches that polypropylene compositions having improved flexibility and toughness at both room temperature and below 0.degree. C., yet retaining a large measure of high temperature properties, are obtained by incorporating into isotactic polypropylene from 5 to 50% of a solvating plasticizer such as mineral oil. Still other compositions of polyolefins and such constituents are described in U.S. Pat. Nos. 3,043,793, and 3,664,981. These patents teach compositions suitable for making resilient, flexible molded articles and transparent films. None of these patents teach the preparation of transparent films which can be rendered translucent by stretching and then transparent by releasing the stretching force.
U.S. Pat. No. 3,308,073 describes a porous film useful as a filter for gasoline and the like fluids which is prepared by heating a mixture of 4 to 40 percent polyolefin and 96 to 60 percent of a hydrocarbon liquid to form a homogeneous liquid, solidifying the mixture in the form of a 5 to 100 mil (125 to 2500 microns) layer, and removing the hydrocarbon liquid by evaporation. There is no teaching that the solidified layer would be a transparent film that could be rendered translucent by stretching. Additionally, because of the low concentration of polyolefin, such film would be extremely weak.